1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method which are configured to provide a so-called color blur reduction process to a color image obtained from the imaging.
2. Description of the Related Art
A color image pickup system generates an originally nonexistent color as a color blur around a bright part on an image due to a chromatic aberration of an imaging optical system. The color blur is likely to occur at part distant from a central wavelength of the imaging optical system, and a visible-light color image pickup system causes artifacts in blue, in red, in purple that is a blend of blue and red in a blur shape. This color blur is also referred to as a purple fringe.
A combination of plural lenses having different dispersions can optically reduce the chromatic aberration to some extent. However, as a small size of a digital camera progresses, high resolution image sensors (or image pickup devices) and smaller optical systems are increasingly demanded, and it is difficult to sufficiently reduce the chromatic aberration only by the optics. Therefore, the image processing is required to reduce an artifact.
The chromatic aberration is roughly classified into a lateral chromatic aberration (chromatic aberration of magnification) and a longitudinal (or axial) chromatic aberration. When the light from a light source images on a focal plane via an imaging optical system, as shown in FIG. 1, the lateral chromatic aberration appears as shifts of imaging positions of a B (Blue) ray having a wavelength of about 450 nm, a G (Green) ray having a wavelength of about 550 nm, and a R (Red) ray having a wavelength of about 650 nm shift in the image surface direction. On the other hand, when the light from a light source images on a focal plane via an imaging optical system, as shown in FIG. 2, the longitudinal chromatic aberration appears as shifts of imaging positions of the RBG rays in the optical-axis direction.
As disclosed in U.S. Pat. No. 6,724,702, the lateral chromatic conversion of a digital imaging system of a primary color system can be corrected through a geometric conversion that applies a different deformation to each color plane of RGB.
On the other hand, the longitudinal chromatic aberration means, in an image that is focused with the G (Green) plane as a central wavelength of the visible light region, for example, that the image is out of focus with respect to the R (Red) plane and B (Blue) plane that are ends of the visible light. This cannot be corrected by the geometric conversion although it is effective to the lateral chromatic aberration. The longitudinal chromatic aberration can be made inconspicuous by lowering the chroma of a color blur area as disclosed in Japanese Patent Laid-Open No. (“JP”) 2001-145117, or can be corrected by applying a different contour emphasis process to each color plane of RGB as disclosed in JP 2003-018407.
However, a chroma lowering process disclosed in JP 2001-145117 is a process that degrades a color of a bright pixel down to a brightness level of another color. This process is effective in reducing unnaturalness by erasing a color of the color blur, but the original colors of a subject can sometimes be also influenced and the image is likely grayish irrespective of the existence of the color blur.
The process disclosed in JP 2003-018407 utilizes deconvolution and its approximate contour emphasis process, and is performed by calculating a filter parameter. However, this process fixes G as a reference, and cannot handle a wide variety of color blurs.
The conventional color blur reduction process always provides the same process to blurs that can differ according to images, and cause an unnatural color depending upon a type of a blur or cannot sufficiently reduce or leave a color blur.